The present invention relates to a friction stir welding method.
A friction stir welding method is a technique in which a round shaft (called a rotary tool) is rotated and inserted into the joint between members to be welded, and the rotating rotary tool is moved along the junction line of the joint, thereby heating, mobilizing and plasticising the material in the welding region, so as to effect a solid-phase welding of the members. The rotary tool comprises a large-diameter portion and the small-diameter portion at the leading end thereof. The welding is performed by inserting the small-diameter portion into the joint between the members to be bonded, such that the end surface of the large-diameter portion comes into contact with the aforementioned members. A screw thread is formed an the small-diameter portion. The end surface of the large-diameter portion on the screw thread side has a concave shape. Moreover, friction stir welding may be performed by positioning members to be bonded between two large-diameter portions of the rotary tool. This technique is disclosed in Japanese Patent No. 2,712,838 (U.S. Pat. No. 5,460,317), and Japanese Patent National Publication of PCT Application No. 9-508073 (EP 0752926B1).
A case of friction stir welding the abutting region formed between two extruded members 10, 20 made of aluminum alloy will be considered with reference to FIG. 4. A trapezoidal concave portion 12 is provided at the end surface of the face plate 11 of the member 10, and a trapezoidal convex portion 22 is provided at the end surface of the face plate 21 of the member 20. The convex portion 22 is inserted into the concave portion 12 when the two members 10, 20 are abutted. The concave portion 12 and the convex portion 22 have a trapezoidal shape to simplify insertion. Also, the end portions of the members 10, 20 are provided with raised portions 13, 23 projecting upwardly. In the case where the members 10, 20 are used to form a car body of a railway car, the length of the members 10, 20 is approximately 20 m. Therefore, the members 10, 20 at the abutting region may each tend to constitute a substantially different plane. In order to prevent such situation, the end surfaces of the abutting region are provided with the concave portion 12 and the convex portion 22 and are fitted against each other.
In such a state, friction stir welding is performed by rotating and inserting a small-diameter portion 51 of a rotary tool 50 from above into the welding joint until the end surface of a large-diameter portion 53 (end surface on the small-diameter portion 51 side) is inserted slightly into the raised portions 13, 23. The rotating rotary tool 50 is then moved along the welding joint in the abutting region, as is illustrated in FIG. 9. Reference number 100 indicates a bed for fixing the members 10, 20.
The rotary tool 50 used for welding is shown in FIG. 10. The rotary tool 50 has a small-diameter portion 51 attached to the leading end of the large-diameter portion 53. The small-diameter portion 51 has a tapered shape with a larger diameter portion located towards the leading end. This is to stir the material in the abutting region of the concave portion 12 and the convex portion 22 sufficiently. The tapered surface of the small-diameter portion 51 is provided with a male screw thread. The end surface 53h of the large-diameter portion 53 is contoured so as to be concave towards the large-diameter portion 53.
The small-diameter portion 51 in the vicinity of the large-diameter portion 53 (the small-diameter portion 51 on the base end side) is not provided with a screw thread. This is because it is difficult to provide the screw thread on the small-diameter portion 51 close to the base end side due to the fact that the small-diameter portion 51 is tapered.
As seen in FIG. 9, during welding, the region of the small-diameter portion 51 without the screw thread is placed at the position of the upper slopes (called sides) 12b, 22b of the trapezoidal concave portion 12 and the trapezoidal convex portion 22. Therefore, the stirring in this region tends to be insufficient. When stirring is insufficient, this is considered to have an undesirable influence on the strength of the weld. In the case where the material to be welded is an aluminum alloy, the surfaces of the members 10, 20 tend to be oxidized. When stirring is insufficient, it is considered that such metal oxide has a bad influence on the strength of the weld.
An object of the present invention is to provide a friction stir welding method that is capable of providing good welding at the abutting region with the concave portion and convex portion fitted together.
The present invention is characterized by the face that friction stir welding is performed by positioning a screw thread on the rotary tool in the fitted region of the concave portion and the convex portion.